If geology is a layer cake, Wharfedale is
a two-slice portion. The upper valley is the Lower Carboniferous
area, roughly north-west of Burnsall, where the Great Scar Limestone
forms a basement to the overlying Yoredale Beds, a 300-metre sequence
of hard limestones, sandstones and soft shales. These strata have
been only slightly tilted, down to the east. To the south-east the
Millstone Grit, of the Upper Carboniferous, begins, with its heather
moorland and hard crags and tors.
Weathering of the Yoredales has produced
the classic stepped profile which can be seen in the valley sides,
with a shelf of limestone, sometimes grassy but often displaying
karst features such as limestone pavement, gorges and sinkholes.
Above it a slope of shale slopes up to the next limestone shelf,
and hills such as Great Whernside and Pen-y-Ghent are crowned with
small caps of Millstone Grit. On the tops of the hills can be found
unexpected ponds - Birks Tarn, Fountains Fell Tarn - where the impermeable
layers hold up little peaty pools. The limestones below contain
corals and brachiopods and also minerals, of which lead once formed
a major local industry.
The well-known Craven fault series, North,
Mid and South, is seen to best advantage a little further west,
south of Malham Moor, and separates the Craven uplands from the
lowlands, but runs out to the east near Burnsall where the Millstone
Grit takes over. A series of reef knolls runs west-east immediately
south of the Mid-Craven fault.
The Great Scar limestone and the Yoredale
layers are noted for extensive cave systems. At Birks Fell caves,
west of Buckden, there are two separate systems which formed during
the ice age. They are roughly parallel to Wharfedale itself but
are otherwise quite independent of present drainage lines. Some
of the country's best twisted stalactites in Britain are found here.
Boreham cave has a remarkably extensive system,
formed by water under pressure and deepened while the glaciers were
carving out Littondale. The upper passages were left fairly dry
but have since accumulated glacial deposits and, in one cave, 'a
display of calcite straw stalactites, unsurpassed in Britain.'
At Brants Gill there is a network of potholes
feeding underground streams - Hull Pot has the largest entrance
in Britain and Pen-y-ghent Pot is the country's finest underground
streamway. The Dow Cave system contains the remarkable Dowbergill
Passage, 10-25 metres high but only 1 metre wide and completely
straight for 1,500 metres. It was produced by solution along a fault
and the stream goes through a spur of Great Whernside, sinking on
one side and rising on the other.
Black Keld has two systems, Langcliffe Pot
and Mossdale Caverns, which include over 20 kilometres of passages.
They are unusual since they are not restricted to the Yoredale strata
but break through the shale and sandstones to the Great Scar limestone
below. These systems are important examples of maze caves.
And at Strans Gill, near Hubberholme, is
'probably the most beautifully decorated cave passage anywhere in
Britain,' with magnificent stalagmites and stalactites.
The surface features are
no less dramatic. In the last glaciation, a local ice cap at the
head of the Dales fed valley-glaciers and produced the classic U-shaped
profiles. There was probably a glacial lake, overlooked by Kilnsey
Crag, and opposite is the narrow dry meltwater channel of Conistone
Dib, with a deeply-incised gorge called the Gurling Trough. To the
south, Dib Scar is a dry former waterfall, similar to the better-known
Malham Cove in Airedale. The glacier scoured the limestone, producing
the pavements which have since weathered into raised 'clints' and
the cracks, 'grikes', between them.
Pen-y-Ghent Gill is another limestone gorge,
cut into the Great Scar Limestone. It contains the Giant's Cave,
of national physiographic significance as the 'finest example in
Britain of cavern collapse in action.'
To the south of this area, running in a line
south of the Mid-Craven fault, is a series of limestone knolls,
green against the darker millstone grit background of Cracoe Fell.
These were laid down in shallow warm seas along a former coastline
and are known here as the Cracoe Reef Knolls.
This is the classic site for the study of
knoll-reefs in the Asbian carboniferous limestone and is a key site
for understanding reef communities and their palaeoecology. At Swinden
Quarry a reef has been dissected by quarrying and its internal structure
revealed. This includes a reef core of thickly-bedded limestones,
rather sparsely populated, but with marginal deposits with an abundant
shelly fauna including large brachiopods, varied gastropods and
occasional bivalves. Intrusive minerals include valuable lead deposits.
Come down the valley and the change of underlying
rock can be seen in the darker stone in the field walls. The Millstone
Grit outcrops at the well-known Cow and Calf Rocks at Ilkley and
forms a rolling dissected plateau. The impermeable nature of the
rock produces blanket bogs and mires, and drier areas have wet and
dry heaths and acid grasslands.
Cayton Gill Beds stretch north from the Washburn
to Masham, muddy sandstones and siltstones which contain many remains
of brachiopods and bivalves. Great Almscliff Crag is one of the
largest and most massive gritstone tors in the Pennines. It is of
importance since it shows the association of sound, and weathered,
bedrock, and is within the limits of the last glaciation, unlike
many similar features further south.
In mid-Wharfedale the coarse sandstones are
known as Addingham Edge and Bramhope Grits. The Otley Shell Beds
can be seen exposed in Otley Chevin and at Great Dib Wood there
is a natural section through two Namurian sandstones with the Otley
Shell Bed sandwiched between. This is one of the youngest horizons
to yield trilobites at surface exposures in Britain. It is rich
in remains of many of the animals that inhabited the Carboniferous
Glacial lakes filled mid-Wharfedale at one
time and deposited sand and gravel, which have been quarried and
now form the basis of the Society's Otley Wetland Reserve, and Ben
Rhydding and Knotford Nook gravel pits.
|Do not assume that sites mentioned here
have public access. Please use only public footpaths or ask
permission for access. Conservation of our wild heritage depends
on the goodwill of landholders - please don't abuse it!
You are here: Home/Wharfedale/Habitats/